2.1. Gene Therapy
Gene therapy is generally understood to refer to techniques designed to deliver functionally active therapeutic genes into targeted cells. Such therapeutic genes may encode proteins that complement genetic deficiencies, cytokines, cell surface membrane proteins or any protein that functions to regulate cell growth and/or differentiation. Such proteins may function intracellularly, for example, by regulating a signalling pathway or transcriptional pathway. Alternatively, the proteins may be secreted by the cell and exert their effect extracellularly.
Initial efforts toward somatic gene therapy have relied on indirect means of introducing genes into tissues, e.g., target cells are removed from the body, transfected or infected with vectors carrying recombinant genes, and reimplanted into the body. These types of techniques are generally referred to as in vitro treatment protocols.
In contrast, direct in vivo gene transfer has recently been achieved with formulations of DNA trapped in liposomes (Ledley et al., 1987); or in proteoliposomes that contain viral envelope receptor proteins (Nicolau et al, 1983); calcium phosphate-coprecipitated DNA (Benvensisty & Reshef, 1986); and DNA coupled to a polylysine-glycoprotein carrier compounds (Wu & Wu, 1988).
In addition, recombinant replication-defective viral vectors have been used to infect cells both in vitro and in vivo. Perhaps the most widely studied viral vectors for use in gene therapy have been the retroviral vectors. The major disadvantages associated with the use of retroviral vectors include the inability of many viral vectors to infect non-dividing cells, problems associated with insertional mutagenesis and potential helper virus production. Recently, attention has turned to the use of other types of recombinant viral vectors such as adenovirus and adeno-associated virus based vectors, that may be used to deliver genes of interest to cells.
In particular, recombinant adeno-associated virus has many features of interest in the field of gene therapy. The vectors are based on a defective, nonpathogenic human parvovirus that can infect both dividing and non-dividing cells without a marked tropism. In addition, the viral genome can stably integrate within the host genome, facilitating long term gene transfer.